CONDITIONS RELATED TO UPWELLING WHICH DETERMINE 

 DISTRIBUTION OF TROPICAL TUNAS OFF WESTERN BAJA CALIFORNIA'^ 



BY MAURICE BLACKBURN, RESEARCH BIOLOGIST 



INSTITUTE OF MARINE RESOURCES, SCRIPPS INSTITUTION OF OCEANOGRAPHY 

 UNIVERSITY OF CALIFORNIA, SAN DIEGO, CALIF. 92037 



ABSTRACT 



Six oceanographic cruises were made oif the west 

 coast of southern Baja California in June through 

 November, 1959-66, and some of the results were com- 

 pared with contemporaneous fishery data on the 

 distribution of yellowfin tuna, Thunnus albacares, 

 and skipjack tuna, Euthynnus pelamis. The object 

 was to test the hypothesis that the tunas generally 

 do not aggregate in waters cooler than 20° C. even 

 when suitable food is abundant, but do aggregate in 

 warmer waters provided that suitable food is abundant. 

 The measure of abundance of suitable food was the 

 concentration of the pelagic red crab Pleuroncodes 

 planipes, a herbivore which is the principal component 

 of the tunas' diet in the Baja California region. 



The results supported the hypothesis very well except 

 on the cruise made in June. Then the coastal upwelling 

 was still strong and some tuna entered waters as cold 

 as 17° C, but no colder, where red crabs were abundant. 

 Areas with temperatures 20° C. or over were very lim- 

 ited, and food was generally scarce in them, although 

 it was plentiful in the extensive areas of upwelled water 

 under 17° C. On each of the other five cruises, which 

 covered the period of decay and disappearance of up- 

 welling, extensive areas contained abundant food at 

 temperatures at and over 20° C. Tunas aggregated in 

 or very near those areas, and nowhere else in the cruise 

 region. Red crabs were most abundant in places where 

 their food, phytoplankton (measured as surface chloro- 

 phyll a), was most plentiful, and in the upwelling 

 season these places were areas of cool upwelled water. 



The tunas aggregated at first around the edges of the 

 cool areas, which were rich in chlorophyll a and red 

 crabs. Later, when surface temperatures in the cores 

 of the cool areas rose past 20° C, the tunas aggregated 

 there as well. Eventually, after all upwelling had ceased, 

 the distribution of surface chlorophyll a, red crabs, and 

 tunas became rather uniform off the Baja California 

 coast. 



These relations are considered to support the follow- 

 ing general statement of tuna ecology, for which there 

 was some prior justification: temperatures set limits 

 of total range, sometimes differently for different 

 species, and food supply determines distribution 

 within the range limits. 



Tuna avoid the Cape San Lucas front when it con- 

 tains water below 20° C, but otherwise the front may 

 have no effect upon them. There is no evidence of aggre- 

 gation of tuna prey in the front. 



As a result of the association of red crabs with phyto- 

 plankton (surface chlorophyll a) tuna generally occur 

 in the parts of the Baja California region where surface 

 chlorophyll a concentrations are relatively high, 

 provided that surface temperatures are not below 20° C. 

 If the region could be thoroughly and frequently 

 monitored for surface temperature and surface chloro- 

 phyll a during a tuna season, areas of probable tuna ag- 

 gregation could be specified. It may eventually be prac- 

 ticable to do the monitoring from ships, aircraft, or 

 satellites. It would not suffice to monitor surface 

 temperature only. 



Most species of tunas liave a wide range in the 

 world's oceans. Their distribution appears to de- 

 pend mainly uix)n two oceanic properties: teniper- 



' Contribution from the Scripps Institution of Oceanography, University 

 of California, San Diego, Calif. 92037. 



2 This work was part of the research of the .STOR (Scripps Tuna Ocean- 

 ography Research) Program. It was supported by the Bureau of Commercial 

 Fisheries under Contracts 14-19-008-9354, 14-17-0007-139, 14-17-0007-221, 

 14-17-0007-306, 14-17-0007-)58, and 14-17-0007-742. Part of the cost of cruise 

 TO-65-1 was provided by the National Science Foundation through a grant 

 in support of the ship operations of the Scripps Institution of Oceanography. 



Published November 1»69. 



FISHERY BULLETIN: VOL. 68, NO. 1 



ature, which sets limits of total range for each 

 species; and standing stock of animals that tuna 

 will eat, which determines distribution within the 

 range limits. This opinion is reasonable, widely 

 held, and supported by a large amount of infonna- 

 tion (Blackburn, 1965). 



Much of the evidence for the hypothesis, how- 

 ever, is indirect. Because tunas are difficult sub- 

 jects for experiments, tuna ecologj' depends upon 



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